d.background is just a wrapper around a simple d.erase call, but it has a name which is more relevant in the context of setting the background color. In other words, when setting a background color, the first call in the series of commands does not have to be d.erase anymore, but it can be d.background, i.e., you start with d.background, not d.erase.

d.mon wx0 does not support d.erase bgcolor, so d.background does not work either. In the main GUI, d.background works (only) as a command layer (same as for d.erase).

The color option is not defined using a standard option because the default is defined for the standard option and d.background cannot have a default color.

Example with an image is included.